Several bacterial proteins capable of binding specific substrates of transport systems have been isolated in recent years. We have studied a galactose-binding protein (GBP) present in Escherichia coli. Both genetic and biochemical evidence indicate that the GBP is part of the system responsible for the active transport of D-galactose and its analogs. Previous results from this laboratory have shown that specific antibodies induce conformational changes which increase the affinity of the GBP for galactose. This effect appears to be related to the binding function of the GBP since it occurs only when galactose is present before adding the antibodies to the GBP. When antibodies are added to the GBP before the galactose, no significant binding is observed. Our working hypothesis is that the transport system consists of a binding protein which acts as a carrier and a second component which, in a manner similar to that of antibody, can increase or decrease the affinity of the carrier for the substrate. A search for such an effector is now feasible since the necessary biochemical tools have been developed in our laboratory. The goal in this proposed research is to isolate and characterize this component of the galactose transport system. From these studies, it should be possible to construct more realistic models for translocation of specific molecules across membranes. The present models are constrained by the fact that only one component, namely a binding protein, has been demonstrated.